An optical star coupler is a device which comprises a plurality of input optical fibers, a coupling region, and a plurality of output optical fibers An optical star coupler typically operates to transmit a fraction of the optical power received at each input fiber to all output fibers. As a result, a star coupler is particularly useful for implementing an optical bus which enables a plurality of terminals to communicate with one another. A typical star coupler is an 8.times.8 device, i.e., it comprises eight input fibers and eight output fibers. However, in a typical application not all of the input fibers receive optical signals and not all of the output fibers are connected to other fibers for transmitting optical signals to remote locations. For example, to provide a 4.times.4 coupler, four of the eight input fibers are not utilized and four of the eight output fibers are not utilized.
Such unused fibers conveying output signals result in undesired reflections which might prove deleterious to the operation of the system in which the coupler is incorporated. Such reflections result from the fiber-air index of refraction mismatch at the ends of the unused fibers. Typically, the index of refraction mismatch at a glass fiber-air interface results in a reflection of four percent of the optical signal
Consider, for example, a 4.times.4 coupler formed originally from an 8.times.8 star coupler having four unused input fibers and four unused output fibers. The optical signal arriving on each of the used input fiber is distributed by the coupling region to all eight output fibers. The radiation distributed to the used output fibers is transmitted via connector assemblies to other fibers for transmission to remote locations. At the ends of the four unused output fibers, reflections take place. The reflected radiation is then distributed by the coupling region to all the input fibers where reflection again takes place at the glass-air interfaces at the ends of the unused input fibers. This reflected radiation is then transmitted by the coupling region back to the output fibers, etc. Thus, in an optical coupler system, the glass-air interfaces at the ends of the unused fibers cause multiple reflections. For this reason, reflections at fiber end-air interfaces may be extremely deleterious to the performance of optical systems.
In view of the above, it is an object of the present invention to provide a termination for the ends of unused optical fibers of devices such as star couplers to eliminate deleterious reflections.
It is a further object of the invention to provide a termination for the ends of input and output fibers of an optical device which enables such fibers to be selectively utilized or not utilized depending on a particular application and which prevents deleterious multiple reflections of the type described above when input or output fibers are not utilized.